dt-bindings: convert rockchip-drm.txt to rockchip-drm.yaml

convert the binding file rockchip-drm.txt to yaml format.
This was tested and verified on ARM and ARM64 with:
make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/display/rockchip/rockchip-drm.yaml
make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/display/rockchip/rockchip-drm.yaml

Changes since v2:
- add a missing ">" sign in maintainers list
- change the licens to GPL-2.0-only
- add "additionalProperties: false"
- change the commit message to conform that it was tested on both ARM and ARM64
Changes since v1:
- fixed worng sign-off
- fixed the path of the $id property to be the path of the yaml file

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121154314.3444-1-dafna.hirschfeld@collabora.com

Documentation/devicetree/bindings/display/rockchip/rockchip-drm.yaml

@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: (GPL-2.0-only)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/rockchip/rockchip-drm.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Rockchip DRM master device
+
+maintainers:
+  - Sandy Huang <hjc@rock-chips.com>
+  - Heiko Stuebner <heiko@sntech.de>
+
+description: |
+  The Rockchip DRM master device is a virtual device needed to list all
+  vop devices or other display interface nodes that comprise the
+  graphics subsystem.
+
+properties:
+  compatible:
+    const: rockchip,display-subsystem
+
+  ports:
+    $ref: /schemas/types.yaml#/definitions/phandle-array
+    description: |
+      Should contain a list of phandles pointing to display interface port
+      of vop devices. vop definitions as defined in
+      Documentation/devicetree/bindings/display/rockchip/rockchip-vop.txt
+
+required:
+  - compatible
+  - ports
+
+additionalProperties: false
+
+examples:
+  - |
+    display-subsystem {
+        compatible = "rockchip,display-subsystem";
+        ports = <&vopl_out>, <&vopb_out>;
+    };